Re: [multi_array] Feature request: constructor and resize from values

From: Eugenio Bargiacchi (svalorzen_at_[hidden])
Date: 2020-11-30 14:27:57

> what could it mean to have a block of memory holding one that has not
been initialized?

I am not proposing this. I am proposing to copy-construct elements, just as
std::vector does. Note that std::vector has the following constructor and
resize functions:

    vector::vector( size_type count, const T& value = T ); // I am skipping
the allocator since it doesn't matter
    void vector::resize( size_type count, const value_type& value );

Note how in both signatures there is an argument, value, that can take an
user-constructed value and simply copy it. Since the user has constructed
the value, it is well formed. This allows you to use std::vector with types
that are not default-constructible. This ofc requires the type to be
copy-constructible, but this can be a lesser restriction than not allowing
user-defined constructors in the first place. Again, the standard library
does this, so it's probably been motivated already.

I do not see a reason why multi_array could not do the same. In multi_array
case, the signature would be in the form:

  multi_array(extents, const T & value);
  resize(extents, const T & value);

I hope this clarifies what my request is. I do not want to use pointers, in
the same way that one would not recommend using pointers when storing
non-default-constructible classes in std::vector.

Best,
Eugenio Bargiacchi

On Wed, Nov 25, 2020 at 8:31 PM DV Henkel-Wallace <gumby_at_[hidden]>
wrote:

> If an object does not have a default constructor what could it mean to
> have a block of memory holding one that has not been initialized?  How
> would your program know if the object had been properly initialized or not?
>
> There are three ways to address this:
>
> 1 - A pointer, as already mentioned.  A null pointer means the spot does
> not refer to ("contain") a valid object -- or any object at all.
>
> 2 - use a variant
>
> 3 - create a default constructor (which you could do by changing the
> signature of your constructor to something like (int = 0) or (int = -1)
> assuming those values are invalid).
>
> In all cases, regardless of semantics or implementation you'll either have
> to check at runtime for a valid object OR, if you know for other reasons
> which locations are valid, use the default constructor with any value you'd
> like.
>
> If you don't want to use pointers there are a couple of other
> possibilities:
>
>
> Date: Sun, 22 Nov 2020 12:06:04 +0100
> From: Eugenio Bargiacchi <svalorzen_at_[hidden]>
>
> Ah, sorry, I thought it would have been automatically clear. Consider this
> example:
>
> class A {
>    public:
>        A(int) {}
> };
>
> boost::multi_array<A,2> array(boost::extents[2][2]); // Compiler error
> boost::multi_array<A,2> array(boost::extents[2][2], A(3)); // What I would
> like
>
> array.resize(boost::extents[2][2]); // Again, compiler error
> array.resize(boost::extents[2][2], A(2)); // What I would like
>
> The variable `array` cannot be constructed in any way, as multi_array will
> try to default-construct it, and that will fail. Trying to simply create
> `array` with its default constructor works (although it will obviously be a
> zero-sized array), but when trying to resize it (to actually store things),
> the same thing will happen.
>
>
>